Optical signal transmission may be used to communicate signals between separated integrated circuit chips to provide inter-chip connections and within components on the same integrated circuit chip to provide inter-chip connections. In many instances, it is necessary to couple an external optical fiber to a photonic device, e.g., a waveguide, of an integrated circuit photonics chip. Such coupling requires precise optical alignment between the optical fiber and the photonic device to maximize optical signal transmission between them.
However, coupling light into and out of a sub-micron integrated photonic device, such as a waveguide, with high efficiency is difficult because of the small waveguide mode size compared with that of an optical fiber.
Such optical coupling is made conventionally through a diffraction grating coupler provided on a planar upper surface of a waveguide which changes the direction of an optical signal passing through the waveguide from being generally parallel to the running length of waveguide to a direction which is out of the waveguide.
FIG. 1 illustrates in cross section an example of a prior art grating coupler. An integrated circuit photonic structure 10 is provided which has a silicon-on-insulator (SOI) substrate having a silicon base 11, a buried oxide (BOX) 13, typically formed of silicon dioxide, formed over silicon base 11, and a silicon fabrication material 26, which is formed into a waveguide core 15. The BOX 13 provides a lower cladding for the silicon waveguide core 15 and a further oxide material 17, which has a flat upper surface 22, is provided as side and an upper cladding for the waveguide core 15. A grating coupler 21 is formed in the upper surface 18 of the waveguide core 15 to direct light passing between the waveguide core 15 and an optical fiber 131. The optical fiber 131 has a core 133 and outer cladding 135.
As shown, light entering into or exiting from the grating coupler 21 in the direction of arrows A is angled along optical axis B relative to the upper surface 22 of upper cladding 17. This angling of light along axis B is an inherent characteristic of grating coupler 21. Depending on the design of the grating coupler 21, including materials used, the optical axis B is at an angle in the range of about 8 to about 12 degrees from a direction normal to the upper surface 22 of photonic structure 10. As a result, if an optical fiber 131 is arranged to be normal to the upper surface of the photonic structure there is a considerable optical signal power loss, as much as 50%, between the grating coupler 21 and optical fiber 131. Thus, to obtain maximum efficiency in the transfer of light between the grating coupler 21 and optical fiber 131, the optical fiber 131 must, as shown, also be angled by a like amount relative to the upper surface 22 of the photonic structure 10. This complicates packaging of the photonic structure 10 as a mechanical angled coupling must be provided for the optical fiber 131. Moreover, the connection between the angled optical fiber 131 and photonic structure 10 typically requires an active alignment system to ensure alignment of the optical fiber 131 to the photonic structure 10 along optical axis B. This adds costs and complexity to the packaging of the photonic structure 10.
What is needed is a grating coupler and method of formation which provides an optical signal which is emitted to or received by an optical grating coupler in a direction substantially normal to the upper surface 22 of the photonic structure 10 to facilitate mechanical coupling with an optical fiber 131.